There has been proposed a system for enabling cipher communication by providing a common secret for both of a valid sender and a valid receiver in advance. In such a system, the sender and the receiver can authenticate each other by obtaining the secret of their counterpart before starting communication, to determine whether or not it corresponds to his own secret (see Patent document 1).
Published Unexamined Patent Application No. 5-227162
[Problems to be Solved by the Invention]
In the system described above, however, it may occur that the sender finds the secret of the counterpart to be invalid after sending the valid secret he has, thus, the valid secret is disclosed to the invalid counterpart.
Thus, the object of the present invention is to provide an authentication device, an authentication system, an authentication method, a program and a recording medium capable of solving the above problem. The object is achieved by combinations of the characteristics described in the independent Claims in the Claims. More advantageous examples of the present invention are Claimed in the dependent Claims.